The JPEG (Joint Photographic Experts Group) format is a popular format for images on the Web because it enables file creators to balance image size and quality. For example, pictures meant for instant display on websites can use 72 dots per inch (DPI), while those meant for printing might use 300 dots per inch. Although you cannot improve the quality of a 72 DPI JPEG by increasing it to 300 DPI, you can reduce the storage space and download needs of a higher-DPI (for example, 600-DPI) image by shrinking it to 300 DPI through the use of several tools.
Browse the free online Third Light Converter. It works completely on the Web and allows you to avoid downloading applications that take up space on your hard drive. It is ideal for use on netbooks or laptops with minimum storage. However, you are limited to file sizes of 20 MB or less. You can adjust picture dimensions, convert to JPEG from several formats and see previews of your changes before committing to them. Unlike many online converters, you don't need to register or send in your email. Simply click the "Upload" and "Download" buttons to process your file.
Download and run a free application like IrfanView. These free applications are good if you need to perform conversions without an Internet connection and need to remove any file size restrictions. The program supports far more formats than Third Light, can process several files through batch processing, adds special effects such as blur and sharpen, can extract icons and lets you paint shapes, circles and arrows on the image. You can also turn JPEGs into slideshows or screensavers, or burn pictures directly to CD. To change an image's DPI in IrfanView, press the "I" key. In the dialog that pops up, enter a new DPI setting, then click "Change." Press "OK," then save your image to save its new DPI setting.
Buy and run a commercial art program such as Adobe Photoshop, arguably the most commonly used image-processing program for personal computers. It includes all the features of the previously mentioned applications, but adds finer control of pictures. You can automatically correct problems like lens-distorted or crooked images, adjust color and tone, remove noise or add granularity, easily select parts of the JPEG to change and apply a multitude of filters without destroying the original. A free trial is available on Adobe's website.